Left: Visitors navigate through a series of Jersey barriers and fences to reach the Washington Monument. The barriers were put in place in August 1998 as a precaution after the United States attacked terrorist targets in Afghanistan and Sudan.

Right: By July 2005, the concrete barriers were replaced by interlocking rings of distant, low granite walls. After an earthquake cracked the top of the monument on Aug. 23, a fence was installed to exclude visitors from the base of the monument.

SOURCES: Staff reports; PHOTOS by Shawn Thew for The Washington Post, left, and Patterson Clark, right - The Washington Post.

Left: A passenger sends his baggage through an X-ray machine at Dulles International Airport.

Right: Passengers must currently remove their shoes when passing through airport security, but that requirement may eventually be lifted.